When Does a Go Variable Become Unreachable, Even Within Its Scope?

When Does a Variable Become Unreachable in Go?

In Go, a variable becomes unreachable when the runtime determines that it is impossible for the program to reference it again. This can occur even if the variable has not yet left its declared scope.

Understanding the Example

Consider the example you provided:

type File struct { d int }
d, err := syscall.Open("/file/path", syscall.O_RDONLY, 0)
p := &FILE{d}
runtime.SetFinalizer(p, func(p *File) { syscall.Close(p.d) })

The p variable points to a struct containing a file descriptor. A finalizer is attached to p to close the file descriptor when p becomes unreachable. However, the last use of p in Go code is when it is passed to syscall.Read().

Why p Becomes Unreachable

The implementation of syscall.Read() may access the file descriptor after the call is initiated. This means that p is used outside of the Go code, which allows the runtime to mark it as unreachable. Even though p has not yet left its scope, it is no longer live within the Go codebase.

Purpose of runtime.KeepAlive()

To prevent this premature marking of p as unreachable, you can use runtime.KeepAlive() as demonstrated in the example:

runtime.KeepAlive(p)

By referencing p in this function call, the runtime is instructed to keep it alive until syscall.Read() returns. This ensures that the file descriptor remains valid for the duration of the syscall.

Conclusion

In Go, a variable can become unreachable when it is no longer referenced in the program's code and is being used externally, even if it has not yet left its declared scope. runtime.KeepAlive() provides a clear and effective way to prevent variables from becoming unreachable prematurely, ensuring the proper execution of finalizers and avoiding unexpected behavior.

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